Makes 1 cup
1/4 cup champagne vinegar
3/4 cup red-chili oil
Kosher salt and freshly ground black pepper
Pinch of red-chili flakes
Place the vinegar in a small bowl and drizzle in the chili oil, whisking until incorporated. Add the salt, pepper, and chili flakes, and blend well.
To serve, slice each piece of tuna about 1/8 inch thick, using a very sharp knife or an electric knife. Make mounds of cucumber slices on 4 chilled plates and surround them with 3 or 4 slices of tuna. Drizzle the red-chili vinaigrette around the tuna.